1. Field of the Invention
The present invention relates to an output circuit and, more particularly, to an output circuit having an overcurrent protection function.
2. Description of a Related Art
An output circuit has a switching element to control on/off of power supply to a load according to input control signals. A transistor such as a power MOS is used as the switching element. If an overcurrent flows in the switching element, it can break down the switching element due to overheating and so on. Thus, the output circuit normally has an overcurrent protection function to protect the switching element from the breakdown.
Techniques for the overcurrent protection function of the switching element are described in Japanese Unexamined Patent Application Publication No. 62-243418 and Japanese Unexamined Patent Application Publication No. 10-107605, for example. According to the former technique, a resistor as a current detector is placed in the emitter side of an output transistor constituting a switching element so as to detect an overcurrent flowing in the output transistor. A capacitor connects the base of the output transistor with the base of another transistor for turning off the output transistor. The capacitor is charged or discharged upon detection of an overcurrent by the current detector, thereby intermittently turning on and off the output transistor to protect the switching element.
Generally in the techniques for protecting a switching element from an overcurrent, including those described in the above conventional arts, a time period from the detection of the overcurrent to the turn-off of the switching element, which is called a shut-down time, is set according to a charge/discharge time of the capacitor, that is, a capacitance value of the capacitor, or a resistance value that determines a charge/discharge current value of the capacitor. The shut-down time set in this way is constant regardless of the degree of abnormality occurring in a load. The overheating of the switching element depends on the value and time of the current flowing through the switching element. Thus, in the case where significant abnormality occurs in the load and a large current flows through the switching element, it is necessary to set the shut-down time short in order to avoid that the switching element is broken down before it is forcibly turned off. If, however, the shut-down time is set short and a threshold current for overcurrent detection (a minimum value of an abnormal current) is set low, the switching element can be undesirably turned off due to a rush current immediately after turn-on even when no abnormality occurs in the load.
It is possible to avoid this problem by increasing the threshold current for overcurrent detection. However, if the overcurrent detection threshold current is set high in order to avoid the forced turn-off of the switching element, it is impossible to effectively protect the switching element from the breakdown due to overheating when the degree of abnormality occurring in a load is low and a current which is higher than a steady current of the load but not as high as the overcurrent detection threshold current keeps flowing through the load since the overheating of the switching element depends on the value and time of the current flowing through the switching element.